The invention disclosed herein relates generally to voltage regulation circuitry, and more particularly to an integrated circuit voltage regulator capable of withstanding high transient voltage excursions such as found in automotive applications.
Electronic systems often include voltage regulators which receive electric current at an unregulated supply voltage and provide a regulated voltage to an electrical load. In modern systems, many of the load circuits requiring electrical power are integrated circuits. Integrated circuits and sensors used in connection therewith require relatively low voltages, typically six volts or less. It has become common in connection with such circuits and sensors to provide voltage regulation by means of integrated circuit voltage regulators either on a separate chip or on the same chip as the integrated circuit being powered.
Monolithic integrated circuits are being used increasingly in automotive applications. The automotive environment provides particularly severe voltage regulation requirements. Voltage transients of 80 or 90 volts of either polarity relative to ground may be present on the power supply conductors as a result of interaction of the alternator and main voltage regulator when the engine is shut down or jump started or a battery cable is lifted or certain other electrical connections are broken. In addition, automotive applications may require operation over a broad range of temperatures.
Thus, a voltage regulator for many automotive electronic circuits must provide a relatively closely regulated low voltage from a supply voltage subject to positive and negative voltage excursions many times the magnitude of the required voltage. Further, it must provide such regulation over a wide temperature range. Finally, cost and reliability are very important in automotive applications. Cost must be minimized, thus requiring a simple reliable circuit design which can be readily manufactured.
A problem encountered with circuits including bipolar transistors is that it is difficult to fabricate such transistors capable of withstanding voltage transients of the magnitude expected in automotive applications by common processes for fabricating monolithic integrated circuits. Accordingly, automotive integrated circuit voltage regulators must include some form of transient voltage protection. Various circuit designs and techniques for providing transient voltage protection are known.
One approach is shown in U.S. Pat. No. 4,319,179 issued to W. Jett, Jr. on March 9, 1982. This approach includes a high voltage sustaining transistor between a regulator pass device and an error amplifier which controls the pass device. High voltage sustaining capability is achieved by means of circuitry which reduces the impedance between the base of the transistor and ground in the event of a moderate transient voltage on the supply conductor. If a large transient voltage occurs, the base is essentially shorted to ground.
Although such circuitry increases the voltage sustaining capability of the transistor, the maximum permissible voltage is still limited to slightly less than the breakdown voltage of the transistor when connected with its base shorted to ground. This voltage limit may not be sufficiently high for many automotive applications. Application of a voltage higher than the breakdown voltage for any appreciable duration results in destruction of the transistor and failure of the regulator. Further, application of a voltage which causes the base electrode to be shorted to ground results in loss of voltage regulation. Even momentary loss of regulation can result in malfunction of logic circuits powered by the regulator. Finally, certain other transistors in the regulator are relatively unprotected, and, under certain conditions, may be subject to destructive voltages.
Many of the limitations of prior art voltage transient protection designs have been avoided in the applicants' voltage regulator which provides a substantially increased breakdown limit for the entire regulator and provides regulation up to the breakdown limit. Under transistor breakdown conditions current through the regulator is limited by internal impedances. Thus, exceeding the transistor breakdown voltage does not necessarily result in destruction of the regulator.